docs: Declare UserArray/TitleArray as Iterator<User>/Iterator<Title>

Declaring the abstract functions and using return type hints helps
phan to understands foreach loop over TitleArray objects

$extraPages = TitleArray::newFromResult( ... );
foreach ( $extraPages as $oldSubpage ) {
$oldSubpage->getDBkey();
}

It also helps phan-seccheck to decide another issue type in
UploadFromFile

Change-Id: I5f9a0dd5578bd9b585fb4d086a3e133a5ca35cf6